Hi, Mary,
I have taken the "detour" you describe. Went from Villaviciosa to Pola de Siero, with a visit to Valdedios, which is quite amazing. When we walked the Norte, there was no albergue in POla de Siero, but now there is, and I hear it is quite nice.
We spent two days in Oviedo, to visit the cathedral, but also to visit the pre-romanesque Asturian jewels up on Naranco, San Miguel de Lillo and Santa Maria del Naranco. They are both incredible monuments. About a half hour walk from the train station.
The day from Oviedo back to the Norte is uninspiring. The first 14 of so kms were fine, as I recall, not on roads, through some wooded areas. In the town of La Posada we lost the arrows and couldn't find any help from anyone -- not in the library, not in the police station. We wound up on the edge of a very busy, narrow road and it was not pleasant, but we had not This was almost 10 years ago now, so maybe people with more recent experiences can comment on whether the way is better marked.
We found that the way into Aviles from the south missed a lot of the heavy industrial areas that you would pass through if you stayed on the norte and walked Villaviciosa to Gijon to Aviles. The actual entrance into Aviles is suburban residential, and the albergue there is in the old part of town and was quite nice when we were there. Don't know what it's like now.
Terry's right that if you drop down to Oviedo, you have a choice to make. Either head back up to the Norte and Aviles, or continue on the Primitivo from Oviedo into Santiago I've done both of these routes and they are both beautiful. It just depends on whether you want to stick to the norte or do a combined Norte/Primitivo. There are some threads on this topic, and the consensus seems to be that either option is a good one, and that you will probably make the decision based on where the people you have formed friendships with want to go.
Here are some now out of date notes on my stages, but maybe they will be of some help:
el-camino-del-norte/topic2829.html
Buen camino, Laurie
p.s. I think that the only thing I would disagree with in your post is the guide's statement that the detour adds "another day" to the norte. It actually adds at least a few days, especially if you take time to see the sights.